Low-Temperature Oxidation and Characterization of Heavy Oil via

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Low-Temperature Oxidation and Characterization of Heavy Oil via Thermal Analysis Wan-Fen Pu,*,#,†,§ Cheng-Dong Yuan,*,#,†,§ Fa-Yang Jin,†,§ Lei Wang,‡ Zhen Qian,‡ Yi-Bo Li,†,§ Dong Li,†,§ and Ya-Fei Chen†,§ †

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, §School of Petroleum and Natural Gas Engineering, Southwest Petroleum University, Chengdu, People’s Republic of China ‡ Research Institute of Engineering Technology, Northwest Oilfield Company, Sinopec, Urumchi, People’s Republic of China ABSTRACT: High pressure air injection (HPAI) without ignition has attracted extensive attention in the air injection based improved oil recovery (IOR) process for light oil reservoirs but was rarely proposed as an IOR process for heavy oil reservoirs. This study aims at evaluating the potential of HPAI without ignition for deep, high pressure, heavy oil reservoirs (Tahe oilfield, Tarim Basin, China). Many low-temperature oxidation (LTO) experiments were carried out to study the oxidation behavior of heavy oil under the reservoir conditions (120 °C, about 30−40 MPa) using an isothermal oxidation reactor. The produced gases were analyzed using gas chromatography for their content of O2, CO2, CO, and hydrocarbon gas (C1−C6) content. The apparent hydrogen/carbon (H/C) and molar ratio of the carbon oxides (m-ratio) were also calculated from effluent gases to analyze oxidation behavior. The effects of quartz, reservoir core (characterized by X-ray diffraction), formation water, and catalyst on LTO were analyzed. Thermogravimetry (TG-DTG) experiments were conducted to characterize the oxidation behavior and kinetics of heavy oil. The Arrhenius method was employed to calculate reaction activation energy. The isothermal oxidation experimental results show that reservoir core, formation water, and catalyst have important influences on LTO. The upgrading of heavy oil occurred in the presence of catalyst. The inflammable coke was formed, and combustion reaction happened at 40 MPa and 120 °C after oxidation for 7 days, which implies heavy oils have a spontaneous combustion potential for HPAI without ignition process in Tahe heavy oil reservoir. Simultaneously, the heavy oil upgrading indicates that HPAI without ignition process in the presence of catalyst is a promising and potential air injection based IOR technique for deep, high pressure, heavy oil reservoirs such as the Tahe oilfield.

1. INTRODUCTION With world oil demand increasing in the face of limited supplies, increasing attention is turning toward nonconventional oil sources as a means to relieve the pressure exerted on conventional stocks.1 Heavy crude oil as one of the alternative sources is perhaps the most readily available to meet short- and long-term needs.2 There are a lot of novel techniques being developed for the extraction of heavy oil. In these techniques, thermal methods with high success rates are more frequently used. These methods mainly include steam flooding,3 cyclic steam stimulation,4 steam assisted gravity drainage (SAGD),5,6 vapor extraction (VAPEX, variation of SAGD, strictly speaking is not a thermal technique),7 in situ combustion (ISC),8 “toeto-heel” air injection (THAI),9 in situ conversion processes with electrical heating, etc. However, most of them are suitable for reservoirs with a relative shallow buried depth due to the facing issues of operation, economy or effectiveness. For deep or extra deep heavy oil reservoirs, such as Tahe oilfield (Tarim Basin, China), the development of heavy oil faces more challenges, and there is little effective technique to date. Therefore, it is urgent to develop an effective method for improved oil recovery (IOR). These reservoirs have a unique character due to high formation temperature; that is, the oils can flow very well and become immovable or hardly movable when they flow out from formation to wellbore. This requires that the technique can achieve downhole or in situ upgrading while providing the driving force. © XXXX American Chemical Society

Air injection can offer unique benefits and technical opportunities for IOR in many candidate reservoirs. Air as driving medium has an endless supply. Air injection potentially combines many of the advantages of immiscible and/or miscible gas injection with those of thermal methods.10 However, the conventional ISC process has limited due to the risk of operation and the difficulty of controlling the process. In recent years, high pressure air injection (HPAI) has proven to be a valuable IOR process.11 It has been successfully implemented for commercial application in the buffalo field and attested to both its technical and economic success.12,13 However, the attention of HPAI has mainly focused on the application for deep, high pressure, light oil reservoirs. For heavy oil reservoirs, the air injection process usually refers to conventional ISC. But HPAI without ignition is never proposed as an IOR process for deep and heavy oil reservoirs. Whether a reservoir would be a good candidate for HPAI without ignition depends on the reactivity of the oil under reservoir conditions. This study aims to explore the feasibility of HPAI without ignition process for high pressure, deep, heavy oil reservoirs. Received: September 22, 2014 Revised: December 30, 2014

A

DOI: 10.1021/ef502135e Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Experimental Strategy and Conditions for Isothermal Oxidation Experimentsa run

temperature (°C)

Vo

Vw

A A1 A2 B

crude oil alone crude oil alone oxidized oil in run A1 crude oil + quartz

30 40 40 30

120 120 120 120

30 30 30 30

C D

crude oil + detritus crude oil + formation water

30 30

120 120

30 30

5.29

E

crude oil + detritus + formation water crude oil + catalyst

30

120

30

5.29

30

120

30

F a

pressure (MPa)

Vq

Vd

oxidation time (h) 72 168 168 72

objective

30

72 72

30

72

to study LTO of used crude oil to study LTO of used crude oil to study LTO of used crude oil to study the effect of porous media on LTO to study the effect of detritus on LTO to study the effect of formation water on LTO under the reservoir conditions

72

to study the effect of catalyst on LTO

30

Vo, Vw, Vq, and Vd are the volume of oil, water, quartz, and detritus, respectively.

Figure 1. Flowchart of the isothermal oxidation experiments. alone, and oil saturated crushed reservoir detritus tests were around 30 mg, 30 mg, and 60 mg, respectively. For oil saturated crushed reservoir detritus test, oil and detritus weight were both around 30 mg. All experiments were performed twice to show the reproducibility. 2.4. Isothermal Oxidation Experiments. Objectives. The objective of isothermal oxidation experiments was to evaluate the potential application of HPAI without ignition process in heavy oil reservoirs in the Tahe oilfield. Table 1 shows the experimental strategy and objective. Experimental Device and Procedures. The isothermal oxidation experiments were conducted in an isothermal oxidation reactor. The body of the reactor was made out of high pressure tubing (ID = 0.80 cm, L = 100 cm) with an effective sample volume of 50.24 mL. The other used apparatuses were described in detail by Zhao et al.14 Figure 1 shows the flowchart of the isothermal oxidation experiments. In the experiments, for runs A−F, known quantities of crude oil, crude oil + quartz, crude oil + detritus, crude oil + formation water, crude oil + detritus + formation water, and crude oil + catalyst were loaded into the reactor, respectively. The detritus/quartz was first loaded into the reactor. Then all the apparatuses were prepared as shown in Figure 1. The crude oil/formation water was injected into the reactor at a constant flow rate of 1.0 mL/min. For run F, the catalyst was mixed with crude oil before crude oil was injected into reactor. A free volume was left to be filled with air at required pressure. The pressure data were periodically collected by the automatic acquisition system. The viscosity of oxidized oil was measured using a Haake MARS III

2. EXPERIMENTAL SECTION 2.1. Materials. Heavy crude oil, formation water and reservoir core were obtained from a block of Tahe oilfield (Tarim Basin, China), Sinopec Northwest Company, China. This block is a carbonate reservoir with a formation temperature of 120 °C. The viscosity and density of crude oil at 1 atm are 6452 mPa·s at 50 °C and 0.947 g/cm3 at 25 °C, respectively. The salinity of formation water is 22 × 104 mg/ L. The viscosity−temperature curve of crude oil was measured, and the experimental data were exponentially regressed (see Section 3.5). The reservoir cores and quartz sands were crushed into 60−80 mesh size. The crushed reservoir core is called detritus in later sections of this paper. 2.2. Characterization of Detritus Sample. Whole rock analysis was conducted on the detritus to obtain rock mineral composition by the X-ray diffraction (XRD) method. XRD patterns were recorded on an XPERT-PRO diffractometer (PANalytical, Netherlands) equipped with an X’Celerator detector and Cu Kα radiation, operating at 40 kV and 40 mA at 25 °C. The diffraction data were collected in the scanning angle (2θ) range of 4−40°, using a step size of 0.0167° and a count time of 15.875 s at each step. 2.3. Thermogravimetric Analysis. NETZSCH STA 409 PC/PG (NETZSCH, Ltd., German) was employed for thermal analysis. The crucible with a type of thermogravimetric-differential thermal analysis (TG/DTA) pan Al2O3 was employed for the normal TG/DTA tests at atmospheric pressure, and the temperature range was 32−700 °C with a heating rate of 10 °C/min. Sample weight of the oil alone, detritus B


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Energy & Fuels Table 2. Effluent Gas Composition in LTO Experiments percent content (mol %) composition

run A

run A1

run A2

run B

run C

run D

run E

run F

C1 C2 C3 IC4 NC4 IC5 NC5 C6 N2 O2 CO2 CO H/C ratio m-ratio oxygen consumption

2.75 0.65 0.09 0.05 0.11 0.05 0.05 0.01 92.27 1.85 0.15 1.97 41.24 0.93 91.19

6.29 0.01 0.14 0.66 0.15 0.07 0.10 0.08 86.378 0.15 5.41 0.56 11.67 0.094 99.29

8.44 0.69 0.00 0.00 0.01 0.01 0.01 0.09 74.81 0.25 15.26 0.43 1.13 0.03 98.81

3.01 0.39 0.21 0.08 0.18 0.09 0.12 0.38 92.5 1.07 1.64 0.33 44.73 0.17 94.90

4.38 0.29 0.14 0.05 0.12 0.08 0.08 0.07 87.57 0.27 6.13 0.82 9.65 0.12 98.71

12.21 0.18 0.01 0.03 0.15 0.01 0.03 0.01 85.96 0.87 0 0.54 162.99 1 95.86

12.38 0.11 0.04 0.02 0.04 0.04 0.05 0.13 86.51 0.58 0 0.1 906.45 1 97.24

6.95 5.01 0 0 0.01 0.04 0.05 0.01 77.34 3.01 7.58 0.00 41.24 0 85.67

independent of the oxygen concentration.21 The model assumes that the rate of mass loss of the total sample is dependent only on the rate constant, the mass of the sample remaining, and the temperature with reaction order of unity. So the equation of Arrhenius-type kinetic model takes the following form.

rotational rheometer (Germany). The HP 6890 Series gas chromatography (Agilent Technologies, Inc.) was used to measure the oxygen consumption as well as CO, CO2, and hydrocarbon gas (C1−C6) content in the effluent gases. The apparent hydrogen/ carbon (H/C) and molar ratio of the carbon oxides (m-ratio) were made to precisely describe the oxidation behavior. The calculations of H/C and m-ratio are useful tools when studying the chemical nature of fuel burned at particular temperatures.10,15 The apparent H/C of the fuel consumed indicates the nature of the fuel burned, for example, the lower the ratio the more coke-like is the deposited fuel. Also, the apparent H/C ratio of less than 3.0 reflects that combustion happened.11,16 The apparent H/C ratio was calculated from the composition of the produced gases. This calculation was based on the assumption that all the oxygen not observed in the exit gas had reacted to form water.10 The apparent H/C ratio is defined as ⎡ 4⎢ ⎣ H = C

γi

( )ν νi

O

dW /dt = kW n

k = A r exp(− E /RT )

dW /dt /w = A r exp(− E /RT ) lg(dW /dt /w) = lg A r − E /2.303RT

(7)

The plot of lg(dW/dt/w) vs 1/T is plotted. The activation energy E and the Arrhenius constant Ar are obtained from the slope (−E/ 2.303R) and intercept of the regression line.

(2)

Together with the H/C ratio and m-ratio, the oxygen consumption is made through the following equation: oxygen consumption (mol%) = [(γi − γO)/γi] × 100

(6)

Taking the logarithm of both sides,

(1)

The m-ratio is determined from effluent gas analysis using the following expression:

m‐ratio = CO/(CO + CO2 )

(5)

where Ar is the Arrhenius constant, E is the activation energy, T is the temperature, and n is the reaction order. Generally, there is no mass transfer limitation; it has been found that oil oxidation is a first order reaction with respect to oil (i.e., “n = 1”).22 Therefore, first-order kinetics is used,

⎤ − CO2 − 0.5CO − γO⎥ ⎦ CO2 + CO

(4)

where dw /dt is the rate of mass change, and k is rate constant which can be expressed,

3. RESULTS AND DISCUSSION 3.1. Oxidation Characteristic of Heavy Oil (for Runs A and C). The produced gas by LTO of heavy oil is composed of CO2, CO, N2, and vaporized/stripped hydrocarbons. The pressure profiles were detected during the reaction. Figure 2 shows a typical pressure curve. At first stage (about before 1.5 days), the pressure declines quickly due to the solubility N2 and O2 in the liquid phase accompanied by a high rate of oxygen consumption. Then, the rate of decline turns slow, and finally the pressure tends to stabilize. However, in the first stage, we can see that the pressure change is fluctuating. This is caused by the combined action of gas solutions, oxygen consumption, and gas production (CO2, CO, and C1−C6). We made a comparison of the oxidation characteristic between Tahe heavy oil and light oil in previous studies.22−24 From Table 3, we can conclude that Tahe heavy oil consumed more oxygen in a much shorter period with less oil volume under similar conditions. This means that Tahe heavy oil has a stronger capacity of oxygen consumption compared with light oil. The concentrations of oxygen were decreased to 0.15−

(3)

In eqs 1, 2, and 3, γi is the concentration of oxygen injected, mol %; γO is the concentration of oxygen in the effluent gases, mol %; νi is the concentration of nitrogen injected, mol %; νO is the concentration of nitrogen in the effluent gases, mol %; CO2 and CO are concentrations of carbon oxides in the effluent gases, mol %. The calculated results of the H/C ratio, m-ratio, and oxygen consumption are shown in Table 2 together with the composition of the effluent gases. 2.5. Kinetic Theory. The non-isothermal kinetic study of weight loss during a combustion process is extremely complex, because of the presence of numerous components and their parallel and consecutive reactions. Many kinetic methods17 (Arrhenius method, Coats− Redfern method, integral method, differential method and Ingraham−Marrier method) all based on Arrhenius kinetic theory have been used for kinetic analysis of the data generated by the TG/DTA experiments. The Arrhenius method has been widely used for combustion kinetic analysis of crude oil,17 oil shales,18 asphaltites,19 lignite,20 etc. In this research, Arrhenius method21 was used. In TG experiments, since the crude oil sample size is small and there is an excess air supply outside the sample pan, the progress of the reaction is C


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Energy & Fuels

Figure 3. Comparison of heavy oil viscosity before and after LTO.

such as hydroperoxides, aldehydes, ketones, and acids. The compounds tend to further react and polymerize with each other, forming heavier, less desirable oil fractions, which can cause a dramatic increase in the oil viscosity. 3.2. Effect of Detritus on Oxidation Behavior. By contrast analysis of the results for runs A, B, and C, we can conclude that the detritus affects the produced gas composition. When the detritus was loaded into the reactor, the level of CO2 in produced gas increased to 6.13% from 0.15% (oil alone). The oxygen consumption increased to 98.71% from 91.19% with the oxygen concentration being further reduced from 1.97% to 0.15%. This signified that the detritus promoted the crude oil oxidation. Simultaneously, the H/C ratio was decreased to a low value (9.65) from a relative high value (41.24). A lower value of H/C ratio means the oxidation reaction was carried onto a deeper level. The lower the ratio the more coke-like is the deposited fuel. The influence of the detritus on crude oil oxidation is mainly up to catalytic effect and specific surface effect. To verify if the detritus has a catalytic effect on oil oxidation, the detritus was replaced with quartz that has the same grain size with the detritus to eliminate specific surface effect, and run B (crude oil + quartz) was carried out. It is generally recognized that quartz has no catalytic effect. Comparing run A and run B, the addition of quartz with only increasing surface area had some influence on the oil oxidation inferred from the level of produced CO2 and O2, but not as much as the detritus did. This indicated that the detritus had a strong catalytic effect. 3.3. Effect of Formation Water on Oxidation Behavior. It was found that the presence of formation water had a strong effect on the produced gas composition. There is absolutely no CO2 in the produced gas (runs D and E) when formation water was added compared to the reaction of crude oil alone. A small amount of CO was produced. However, the oxygen consumption was increased to 95.86% (run D) and 97.24% (run E) from 91.19% (run A) in the presence of formation water. The presence of formation water yielded high apparent

Figure 2. Typical pressure curve for heavy oil oxidation obtained in LTO experiments.

1.85% from 21% in 3 days, which indicated nearly complete oxygen consumption occurred. Usually, a specified 3−5% level of oxygen concentration is considered as the safe level in a practical application.16 Only 0.15−1.85% oxygen remained in 3 days indicating Tahe heavy oil could effectively consume oxygen and restrict oxygen concentration at the production wells to a safe level. The high oxygen consumption of Tahe heavy oil can ensure that the oxygen in the injected air is totally consumed or reduced to a sufficiently low level by LTO when the HPAI process is implemented in Tahe heavy oil reservoirs. In spite of a strong capacity for oxygen consumption, Tahe heavy oil gave an ordinary level of produced CO2 (0.15% for oil alone, 6.13% for oil + detritus) at 30 MPa for 3 days. Of the total oxygen consumed, most participated in oxygen addition reactions instead of being converted to carbon oxides. Generally, two possible reaction pathways exist between the oxygen and the hydrocarbons in low temperature range: oxygen addition and bond scission.11,16 For light oil, both bond scission and oxygen addition reactions may occur below 300 °C, and there is most likely a competition between the two types of reactions at these temperature conditions.25 For light oil, the bond scission reactions are the favored reaction path.11 Therefore, LTO of light oil produces more carbon oxides, and there is little change in the oil viscosity.24 However, the oxidation of heavy oil at the relatively low temperatures is very different from the LTO of light oil. In this study, the oil viscosity after the oxidation showed an increase trend in all the experiments (see Figure 3), except for run F (crude oil + catalyst). It is believed that oxygen addition reactions appear to dominate in these experiments. This is also verified by the high H/C ratio (41.24) and m-ratio (0.93). In this scenario, oxygen atoms are chemically bound into the molecular structure of the liquid hydrocarbons, producing various oxygenated compounds

Table 3. Comparison of Crude Oil Oxidation Parameters Between Tahe Heavy Oil and Light Oil from the Literatures API° Ren et al.22 Greaves et al.23 Ren et al.24 Tahe heavy oil Tahe heavy oil

39 39 37 17.92 17.92

run type oil oil oil oil oil

alone + reservoir core + reservoir core alone + detritus

Vo (mL)

pressure (MPa)

temp (°C)

run duration (h)

final gas composition (O2, CO2, CO)

55 20 59 30 30

25.0 24.5 22.8 31.2 29.8

120 117 102−130 120 120

135 120 140 72 72

9.8%, 2.5%, 0.6% 12.8%, 2.4%, 0.4% 2.4%, 7.4%, 1.0% 1.85%, 0.15%, 1.97% 0.27%, 6.13%, 0.82%

D


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Energy & Fuels H/C ratios. All of these changes give an indication that the oxygen addition dominated under this condition. Oxygen was reacted to form the complex oxygenated compounds, and no breakdown of the oxygenated compounds into carbon dioxide occurred. It had shown that the reactions producing CO2 and CO are independent and have the same reaction order with respect to oxygen concentration and the same activation energies.26−28 CO is not further oxidized to CO2 in the experiments. Therefore, it is believed that formation water mainly inhibit the reaction producing CO2. The result is inconsistent with Lee et al.29 They reported that the presence of water decreased the viscosity of the LTO products and increased the amount of CO2. It is believed that the difference results from the used water in the experiments. In their study, the distilled water was used. In this study, the formation water with a salinity of 22 × 104 mg/L which contained various mineral elements may have an influence on the crude oil oxidation. However, according to the produced gas composition, the level of C1 was greatly increased (12.21% for run D, 12.38% for run E) in the presence of formation water compared with that of run A (2.75%) and run C (4.38%). This demonstrates the formation water enhanced the vaporization of light fractions of crude oil. The influence of water on the distillation behavior of oils also was observed by Barzin et al.25,30,31 Vaporization of oil was recognized as an important mechanism associated with kinetics of air injection process. The presence of water increases the richness of the fuel in the vapor phase. 3.4. Observation of Fuel Deposition for Further Oxidation. We further increased the pressure to 40 MPa and run duration to 168 h of the oxidation experiments for crude oil alone in run A1. The oxidation consumption was increased from 91.19% (run A) to 99.29% (run A1). And the H/C ratio was decreased to 11.67 (run A1) from 41.24 (run A). The level of CO2 was also improved at the same time. Interestingly, we found coke (see Figure 4) formed in the

the apparent H/C ratio of less than 3.0 and the fractional conversion of reacted O2 to carbon oxides reacted of greater than 50% demonstrate the oxidation kinetics are operating in the bond scission/combustion mode. Montes et al.32 also reported that a presence in excess of 12% CO2 is the combustion-like values based on field data and lab results. All the levels of above parameters are a good indication of combustion as the primary reaction path in run A2. 3.5. Effect of Catalyst on Heavy Oil Oxidation. Different concentrations (0.04% and 0.08%) of oil-soluble catalyst cobalt naphthenate were added tocatalyze crude oil oxidation. The viscosity of the oxidized crude oil was measured and is shown in Figure 5. The viscosity was greatly decreased especially for the

Figure 5. Comparison of the viscosity for original heavy oil and the oxidized heavy oil in the presence of catalyst.

viscosity below 80 °C, which indicates the catalyst has the ability to upgrade Tahe heavy oil by LTO. The higher upgrading was achieved with a larger catalyst loading of 0.08%, which confirms that the presence and loading of catalysts are beneficial for improving the quality of the oxidized crude oil. In the presence of catalyst, it is possible that catalysis is relevant to free radical with a high reactivity. The oxidation reaction can be shown through two mechanisms according to Sarma et al.,33 as shown below:

R·+O2 → RO2 · R·+O2 → R′ + HO2 ·

(where R′ is an olefin) Once RO2· is formed, many reaction pathways are open to it, such as hydrogen abstraction, addition to unsaturated molecules, and decomposition to aldehydes, ketones, and/or alcohols. These are exothermic processes, wherein highly reactive free radicals can be generated. As the reaction progresses and conditions change, HO2· is accumulated and will become involved in abstraction reactions to yield hydrogen peroxide, which can be further decomposed into OH· radicals. The catalysis is believed to be related to these highly reactive free radicals. However, the reaction system is particularly complex and still unclear. The reaction mechanism as well as catalytic mechanism requires further study. For many heavy oil reservoirs, crude oil is moveable in the reservoir. However, when the crude oil discharges from reservoir formation and into the wellbore, the flow of crude oil becomes very difficult, even immoveable. This problem is one of the most pressing issues facing heavy oil operators worldwide. The great decrease of

Figure 4. Formed coke and its combustion at room temperature (25 °C) and 1 atm.

reactor after oxidation. The coke is close to carbon in chemical characteristics, which is the fuel for sustaining oil combustion. We carried out an ignition test on the formed coke at room temperature (25 °C) and 1 atm. It was found that it is very easy to burn. The oxidized oil besides the formed coke was put back into the reactor for continued oxidation under the same conditions with run A1. Almost complete oxygen consumption (98.81%) and high levels of produced CO2 (15.26%) were obtained at 40 MPa and 120 °C. The fractional conversion of reacted O2 to carbon oxides is up to 78%. Also, a low apparent H/C ratio of 1.13 was obtained. According to Moore et al.,11 E


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called fuel deposition (FD). FD was believed to be related to negative temperature coefficient behavior in gas-phase hydrocarbon oxidation as the oxidation mechanism of heavy oil is consistent with that of gas-phase hydrocarbon in essence, both of which follow the oxidation mechanism of hydrocarbons. The oxidation of hydrocarbons at relatively low temperatures is a complex process involving degenerate free radical chain branching reactions. FD represents a transition in the oxidation chemistry from a low temperature mechanism to a higher temperature mechanism. Wilk et al.34 indicated that the negative temperature coefficient and the change in mechanism are due to the competition between reactions involving the addition of O2 to the alkyl radical forming the alkylperoxy radical and reactions involving the abstraction of hydrogen from the alkyl radical by O2 to form the conjugate alkene and the hydroperoxyl radical. FD has an important influence on HTO. In the reservoir, combustion under HTO may not be sustainable provided the oil does not have sufficient fuel load.16 The final reaction for crude oil alone occurring at temperatures in the 498−680 °C range (505−640 °C for crude oil + detritus) was known as high temperature oxidation (HTO). In the HTO stage, mainly traditional complete combustion reaction occurred. The oxygen reacts with the hydrocarbon molecules to principally larger quantities of CO2, water, and heat. The three similar reaction intervals were also observed in the crude oil oxidation experiments conducted by Kök et al.17,21,35 However, in their studies, the FD as a transition stage was not obvious in the TG/DTA curves. The kinetic parameters of the FD were also hard to obtain from kinetic parameter calculation curves. For many crude oil oxidation experiments,17,35,36 only the kinetic parameters of LTO and HTO were calculated. In this study, we observed clear FD regions in TG/DTG curves, and the kinetic parameters of FD were calculated well from the fitting curves (the kinetic analyses will be presented in section 3.7). The difference in the FD stage among the oxidation experiments of different oils mainly depends on chemical composition of the crude oil which can affect the amount and deposition rate of fuel. The addition of detritus slightly changed the shape of the TG/DTG curves. In addition, we can find that the temperature range of the FD stage (365−505 °C) of crude oil + detritus was wider compared with that of crude oil alone (410−498 °C), which means the detritus can extend the FD region. At the same time, the LTO region was narrowed, and the LTO was finished in a lower temperature range (32−365 °C for crude oil + detritus). On the one hand, the influence of the detritus on the FD stage can be attributed to the surface area of the detritus.37 In the absence of a matrix in the TG/DTA experiments, fuel may be deposited on the surface of the crucible when oil alone reacted with air. However, a solid matrix with more solid surface was provided when detritus was added. There should be an appreciable increase in the amount of fuel deposited on the matrix. On the other hand, it also related to the mineral composition of the detritus. The influence of the detritus on the crude oil oxidation kinetics will continue to be discussed in section 3.7. 3.7. Kinetic Analysis. The activation energies (E) of three reaction regions (LTO, FD, and HTO) for crude oil alone and crude oil + detritus were determined using Arrhenius methods (Figures 8 and 9), and the results are given in Table 4. The activation energies obtained with the crude oil alone were compared with that acquired in the presence of detritus. Detritus has little effect on the LTO stage. The FD activation

crude oil viscosity after oxidation in the presence of catalyst is very favorable for the flow of crude oil in wellbore and surface gathering pipelines. This gives us an indication that the air injection process without ignition in the presence of catalyst has potential application for the Tahe heavy oil reservoir. 3.6. Comparison of Combustion Characteristic for Heavy Oil Alone and Crude oil + Detritus by TG/DTG Analysis. In TG-DTA experiments, for both crude oil alone and crude oil + detritus in oxidation with air, three distinct reaction regions were identified (Figures 6 and 7) known as low

Figure 6. TG curves for heavy oil, detritus, and oil + detritus in atmosphere environment.

Figure 7. DTG curves for heavy oil, detritus, and oil + detritus in atmosphere environment.

temperature oxidation (LTO), fuel deposition (FD), and high temperature oxidation (HTO).17 The first reaction occurring in the range of 32−410 °C for crude oil alone (32−365 °C for crude oil + detritus) was called LTO, which presumably produces aldehydes, ketones, acids, hydroperoxides, and small quantities of carbon oxides. In this region, two distinct peak valleys (approximately 80−150 °C) observed from both the two DTG curves of crude oil alone and crude oil + detritus were related to the evaporation of light hydrocarbons. The second reaction region occurring in the 410−498 °C range for crude oil alone (365−505 °C for crude oil + detritus) was F


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sufficient fuel is not deposited.39 The fuel is easier to react with air. The more the deposited fuel, the easier the combustion conducted in HTO stage. Third, the catalytic effect of the detritus leads to the reduction of the activation energy. The mineral composition, clay, and metallic content of the detritus besides its surface also have been proven to be the effective catalyst for the crude oil oxidation.40 In this study, the detritus predominantly contains calcite (97.95%) and quartz (2.05%) and hardly clay. Figure 10 and Table 5 show the XRD results of

Figure 8. Kinetic parameters calculation for heavy oil alone by the Arrhenius method.

Figure 10. XRD Results of the detritus for whole rock analysis.

the detritus for whole rock analysis. It is usually considered that quartz has little catalytic effect except for its surface effect. Therefore, it is believed that the catalytic effect comes from calcite. Furthermore, the catalytic effect is more likely related to the metallic ions, especially the transition metal in calcite, rather than the calcite itself. The reason for a significant reduction by calcite needs to be further studied. The spontaneous ignition is expected in the air injection without ignition technique into deep, high pressure, high temperature, heavy oil reservoirs. Sufficient fuel and smooth combustion are very important for autoignition. This would give an implication that for a field situation the effect of the detritus on crude oil oxidation would be very favorable for HPAI process.

Figure 9. Kinetic parameters calculation for heavy oil + detritus by the Arrhenius method.

energy is decreased from 105.14 KJ·mol−1 to 102.59 KJ·mol−1 with the addition of detritus. The HTO activation energy is strongly reduced and is down to 80.27 KJ·mol−1 from 142.67 KJ·mol−1 when detritus is added. This means the detritus has a strong effect on the HTO kinetics. The similar influence rules were observed by Greaves et al.38 as they investigated that how chalk affects the light crude oil oxidation kinetics. In their experiments, chalk has little or no effect on the LTO stage, and the HTO activation energy is reduced to 11.90 KJ·mol−1 from 42.5 KJ·mol−1 and 44 KJ·mol−1. The influence of the detritus on crude oil oxidation can be attributed to the catalytic effect and specific surface effect. The strong effect of the detritus on HTO is believed to be affected by three factors. First, the specific surface effect of the detritus may have an influence on the HTO stage. Second, the deposited fuel is favorable for HTO. The detritus yielded a large amount of fuel because of its surface effect. It has been proved that in situ combustion process is not feasible if

4. CONCLUSION (1) Heavy oil had a stronger capacity of oxygen consumption compared with light oil. A near complete oxygen consumption was achieved at about 30 MPa in 3 days and the oxygen addition reaction appeared to dominate. (2) When pressure and run duration were increased, the coke, which is close to carbon in chemical characteristics, was observed at 40 MPa and 120 °C. The formed coke is flammable at room temperature (25 °C) and 1 atm. In the further oxidation experiment of the oxidized oil and coke, combustion occurred as the primary reaction path, which is supported by high levels of produced CO2 (15.26%), the fractional

Table 4. Oxidation Kinetic Parameters of Crude Oil Alone and Crude Oil + Detritus activation energy (KJ·mol−1)

slope of fitted line

Arrhenius constant (min−1)

sample

LTO

FD

HTO

LTO

FD

HTO

LTO

FD

HTO

crude oil crude oil + detritus

−791.06 −788.70

−5491.39 −5358.11

−7451.33 −4192.40

15.15 15.10

105.14 102.59

142.67 80.27

0.65 0.22

4.47 × 106 9.12 × 105

1.74 × 108 2.04 × 103

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Article

Energy & Fuels Table 5. XRD Analysis Results of the Detritus for Whole Rock Analysis minerals (wt %) sample detritus

clay minerals 0.00

quartz 2.05

orthose 0.00

plagioclase 0.00

AUTHOR INFORMATION

Corresponding Authors

*(W.-F.P.) Tel.: +86-028-83032040. E-mail: megycd@163. com. *(C.-D.Y.). E-mail: [email protected]. Author Contributions #

W.-F.P. and C.-D.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Sinopec Northwest Company (China) for the financial support of this research and permission to publish this paper. This research is financially supported by Key State Science and Technology Project of Large Gas Fields and Coalbed Methane, China (No. 2011ZX05049-04-04HZ). This research is also supported by scientific research project of education department (14ZB0042) and science and technology innovation talent project (2014-070), Sichuan province.

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dolomite 0.00

pyrite 0.00

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conversion of reacted O2 to carbon oxides (78%), and a low apparent H/C ratio of 1.13. The occurrence of combustion implies that Tahe heavy oil has a spontaneous combustion potential for HPAI without ignition process. (3) The detritus had a strong catalytic effect on crude oil oxidation. The presence of detritus extended the temperature range of the FD stage and narrowed the LTO reaction region. Detritus strongly reduced the activation energy of the HTO. The calcite in detritus is believed to be the major reason that detritus has a strong catalytic effect on crude oil oxidation. (4) The presence of formation water had an effect on the produced gas composition, and there is no CO2 produced. Formation water may inhibit the reaction producing CO2. The effect of formation water may be related to various mineral elements existing in formation water. Simultaneously, the formation water enhanced the vaporization of light fractions of crude oil. (5) The presence of catalyst is beneficial in improving the quality of the oxidized oil. The catalyst can greatly decrease the viscosity of the oxidized oil and upgrade Tahe heavy oil. The higher upgrading was achieved with a larger catalyst loading. HPAI without ignition in the presence of catalyst has potential application for the Tahe heavy oil reservoir. Cyclic air simulation in the presence of catalyst probably would be a new choice for IOR.

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calcite 97.95

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Low-Temperature Oxidation and Characterization of Heavy Oil via

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